


fundamentals

by kinkykawa



Series: youngblood (miyacest one-shots) [1]
Category: Haikyuu!!
Genre: (somewhat), Alternate Universe - Canon Divergence, Character Study, Clothed Sex, Dead Dove: Do Not Eat, Frottage, Hand Jobs, Hurt/Comfort, Incest, Introspection, M/M, Miyachiincest, Twincest, moderate angst
Language: English
Status: Completed
Published: 2020-02-13
Updated: 2020-02-13
Packaged: 2021-02-27 21:53:41
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,313
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22692856
Author URL: https://archiveofourown.org/users/kinkykawa/pseuds/kinkykawa
Summary: If Osamu had said no to volleyball, Atsumu would have conceded.But his twin says yes, and Atsumu smiles the smile that he reserves for Osamu and Osamu only.
Relationships: Miya Atsumu/Miya Osamu
Series: youngblood (miyacest one-shots) [1]
Series URL: https://archiveofourown.org/series/1933474
Comments: 26
Kudos: 684





	fundamentals

**Author's Note:**

> i have never written miya atsumu or osamu before this fic but the idea wouldn't fckn leave me alone so FINE HERE IT IS i've somehow written 4k of introspective miyachiincest to satisfy the stupid itch nagging the back of my mind. i hope my brain is happy.
> 
> (if you know what "miyachiincest" is referencing please collect 200 when you pass go for your veteran's discount)
> 
> if you wanna just read miya twins introspection feel free to stop after the train station scene.
> 
> listen. this is posted to the anonymous collection for a reason. if you recognize me (and some people possibly will because my writing style has very distinct markers, fml) either don't say anything or find me on twitter to PRIVATELY say that you liked it. i do love these two tho and i wish we had more content of them together being little shits.
> 
> if you DON'T recognize me and want to talk about things or toss a coin to a witcher then hey my twitter's in the end notes, come say hi XD
> 
> fic sort of edited but i'll fix anything in retrospect.

* * *

There exist three fundamental truths in Miya Osamu’s life:

> One, that he is older than Atsumu by twelve minutes and twenty-three seconds;
> 
> Two, that Atsumu has the worse attitude between them, shameless and precocious and cheeky to a fault; and,
> 
> Three, that Atsumu loves volleyball more than he does.

That third fact is something he has long accepted and never resented. He doesn’t know when he’d begun realizing that volleyball was different for Atsumu — that _Atsumu_ was different when he played volleyball, when he stepped onto a court — but he knows now that it is true. There is an innate hunger inside his twin that simply doesn’t exist inside Osamu, but for him this is fine.

The people around him say he’s not living up to his full potential, that he’d be just as good (if not better) than Atsumu if he put in the same effort, but it’s fine.

Osamu isn’t playing volleyball for the hunger of it, anyway.

(That’s not to say he doesn’t like volleyball, because he does. He knows well the thrill of a clean spike, the delight in a well-timed dig. He hates losing, hates the feeling of being shut down or caught out. He goes to training and runs on their off-days and puts in the work to fine-tune his spike. His best hits always come off of Atsumu’s tosses.

But it’s a fundamental truth, really: Atsumu wants volleyball more.

He’s not better than Osamu — not more skilled, doesn’t have better technique — but he wants it more.)

When they are six, Osamu and Atsumu have a fight over a Metamon plushie that their older cousin had won in an arcade. Atsumu surrenders eventually but he goes down screaming, standing in the middle of the sidewalk and wailing his displeasure. Osamu doesn’t really care and just wants to go home, but their mother bends down and gives him a gentle smile.

“Osamu,” she says, the sweet-bow of her lips, “you’re his older brother, see? You’re the same age, but you’re also older, so you should take care of him.”

Osamu pouts and hugs the plushie tighter. Their mother laughs, ruffling his hair.

“The two of you will have to learn to share with each other, you know? You’re twins after all.” She nudges his shoulder, turning him towards his still-sulking brother. “You were born for each other.”

(Odd, how of all the things his mother says, that’s what sticks to him the most.)

Osamu frowns, watching Atsumu swipe at his eyes, all reddened cheeks and gross sniffles. He glances back at his mother, who nods encouragingly. Then Osamu _hmphs_ and shoves the plushie into Atsumu’s face.

“Here,” he mumbles. “We can share.”

(Thinking back, two things about this moment stick to him. The first is his mother saying _you were born for each other_ ; the second is the way Atsumu peeks around the plushie to give Osamu a wide, sunny smile.)

Atsumu is not inherently better than Osamu at volleyball, but he wants it more. And from that moment on the sidewalk at six years old, Osamu has learned he is helpless to give what Atsumu wants. Whether it’s the clothes in his closet (although not without plenty of grumbling and sniping and occasional counter-theft); the pudding in the fridge (at least, until Osamu starts stocking the flavors Atsumu hates, grinning as his twin scrunches up his face); the new quick set that Atsumu wants to try for the Spring Interhigh—

When they are nine years old, Atsumu wants to try playing volleyball. He looks at Osamu with wide eyes, and Osamu sighs.

It’s not as if he _doesn’t_ want to try, either, anyway.

This is not to say that Osamu is the only one who gives, though. Atsumu may be selfish and willful, but he isn’t as self-absorbed as people make him out to be. Osamu simply asks less, so Atsumu has to work out for himself what his twin wants and how to give it. Peers and adults think Atsumu is the prideful one between them, but Osamu is proud and stubborn in his own right. In many ways he reminds Atsumu of a stray cat — he won’t take gifts unless he can make it seem like he’s doing you a favor by accepting; he only shows affection in roundabout ways. 

So Atsumu will take Osamu’s pudding but stock his favorite cherry sodas on the bottom shelf. He’ll take Osamu’s clothes but do all the laundry when he (eventually) returns them. He leaves yokan on a plate on his brother’s desk without comment and doesn’t wait for the thank you.

If Osamu had said no to volleyball, Atsumu would have conceded.

But his twin says yes, and Atsumu smiles the smile that he reserves for Osamu and Osamu only.

_You were born for each other,_ their mother says, and both of them grow up with this phrase lodged between their ribs. It nudges against their lungs every time they breathe. They don’t isolate themselves — with Atsumu’s personality, it would be impossible — but there exists in their lives an invisible-but-real divide. Osamu is, and Atsumu is, and they are. There is an Osamu and an Atsumu that exist only for each other.

When Atsumu has bad dreams that end in all his ambitions falling through his fingers, he crawls into Osamu’s bed and presses his forehead to his twin’s collarbone. Osamu slides his hands under Atsumu’s shirt, skates his palms up the bare skin of his back, brushes an open mouth over his hair. He lies there and lets Atsumu quiet himself to the muted thud of Osamu’s heartbeat.

When Osamu is frustrated badly enough that his hands shake and his lips pull back in a snarl, Atsumu takes his brother’s hands and cradles them, skimming bare or bruised knuckles with his lips and letting Osamu dig half-moons into practice-worn skin. He sits and waits until Osamu uncurls himself from the wall, from where his face is buried between his knees; until Osamu is breathing easy again.

When Atsumu talks about himself in the future, with volleyball. When Osamu sits beside his brother on his bed and reads aloud, warmth bleeding between their pressed-together thighs. When Atsumu drags his brother onto the balcony to watch a meteor shower. When Osamu tosses a blanket over his brother while he naps on the couch.

They grow up bantering and bickering and biting back at each other with all manner of sarcastic and hurtful words, but there is a fourth fundamental truth that they share.

 _You were born for me,_ they will think, in the quieter moments. _You were born for me, you are mine._

There is, for them, no moment of twisted and selfish realization. These touches have always existed between them, evolving as they grow older. They have belonged to each other from the moment Atsumu followed Osamu into the world.

It is simple, really. Irrefutable, inevitable, irredeemable.

When they reach high school, most of their days start like this:

“Wake up,” Osamu deadpans, kicking the base of Atsumu’s bed loudly. When his twin just groans and rolls over, he does it again, harder.

Atsumu burrows further beneath the blankets and Osamu shrugs. He’d tried; he’s not responsible for Atsumu sleeping through his four alarms and refusing to get up. He leaves the bedroom door open when he leaves.

He’s waiting for the train, eating a taiyaki he’d stopped for on the way, when Atsumu stumbles onto the platform.

“Your tie is a mess,” Osamu says with his mouth full. He swallows, then takes another bite.

“I told ya t’ wake me up!” Atsumu whines, trying to fix his shirt buttons and cuffs at the same time.

One of Osamu’s eyebrows goes up. “I did. Twice.”

“Ya didn’t even _try,_ ” Atsumu protests, glaring. He’s right, of course, but Osamu isn’t going to give him the satisfaction. It’s not his fault Atsumu sleeps like the dead.

“Maybe actually wake up next time,” he points out, checking the time on his phone. When he glances back, Atsumu is pouting at him. “What?”

His twin shamelessly opens his mouth with a prolonged and expectant _aaaah_. Osamu looks at him for eight seconds, then shoves the rest of his taiyaki into his mouth. The extreme effort to chew that much pastry and filling is worth the offended, slightly disgusted look on Atsumu’s face.

He leaves Atsumu sulking on the platform and boards the train, taking the last free seat and ignoring the other boy’s glares. He ignores it too when Atsumu starts to chat up another one of their schoolmates, a cute girl from a neighboring class who laughs like the sound of coins falling. But later, after the train has reached their stop and they’ve exited, after they’ve entered campus — later, Atsumu gives a scalpel-pointed smile and sidles up to Osamu, knocking his shoulder against his brother’s.

“Buy me a strawberry milk,” he says, and Osamu rolls his eyes.

“Die,” he mutters, and Atsumu laughs.

(That laugh is why Osamu responds the way he does, of course, and Atsumu does his best to make Osamu say those things in turn.)

The first time Osamu kisses Atsumu is after they lose to Itachiyama in their first Interhigh.

He’s never seen his brother take a loss so hard before — although Atsumu always takes losses hard, seething for days and redoing every faulty play in his head. But Atsumu is never like this — never standing with teeth bared, fists pressed to the locker door, back hunched over. The rest of the team has filtered out of the club room, tired and listless, leaving Osamu to watch the way his twin’s shoulders shake.

“‘Tsumu,” he says, and Atsumu crumples.

“I couldn’t,” he gasps, sinking to the floor. “I couldn’t.”

(He doesn’t complete the sentence, he can’t. Osamu hears him anyway.)

“No,” he says. Osamu wills his feet to move and carry him over to the half of himself that is wrecked. “Not you, it ain’t just you.”

(He cannot do gentle. Not in words, not in most things. Osamu is not made for soft edges and softer actions. He thinks Atsumu doesn’t need gentle, but he isn’t sure, not in this.)

“It doesn’t.” Atsumu makes a frustrated noise and shakes his head, gripping his upper arms so hard Osamu worries for his jacket. “I just.”

“Then blame me too.” Osamu pries Atsumu’s hands from their death grip and predictably, his twin struggles. Atsumu tries to pull back but Osamu will not let him. “I fucked up too. I’m just as much at fault in this, we all are.” _Blame me too,_ he thinks, _call me out like you always do, like you do with everyone else. Tell me what I did wrong._

 _We can share this too,_ Osamu thinks, and tugs his brother forward.

The first press of their mouths is less of a kiss and more of a desperation. Osamu’s parted lips graze mostly Atsumu’s chin, and they’re both clumsy in their hurt. But Osamu tugs on Atsumu again, tilting his head and threading one hand through his twin’s sweat-matted hair, and this time he catches Atsumu’s exhale between his teeth.

He kisses Atsumu to quiet him, to gentle the wild half-sobs in his throat. He kisses Atsumu to share in the frustration and resentment. He kisses Atsumu because his brother is never more beautiful than he is on a volleyball court. He kisses Atsumu because it is just one more touch between them.

He kisses Atsumu because his brother is here and he wants to.

Why Atsumu kisses him back, he doesn’t know, but Osamu will take it.

They sit there together, tangled with each other, until well after Atsumu’s breaths taper out into small hitches and sniffles. Little by little, Atsumu’s hands soften in Osamu’s hold. A while later and shy fingers sneak between Osamu’s, hesitating just a little before gripping tight.

“Fine,” Atsumu says, and there’s the wryness that Osamu loves and finds irritating. “S’all your fault we lost.”

Osamu snorts before he can stop himself, and just like that, they’re both laughing. They topple to the floor together, and Atsumu in his arms, against his chest, feels like home.

“Fuck you,” Osamu replies, and Atsumu smiles like the sun.

Atsumu learns that Osamu is quitting volleyball after high school on the same day that his brother retires from the Inarizaki volleyball team. Eight days ago they’d lost to Itachiyama in the Spring High semifinals. The third years have formally stepped down from the club, and Atsumu is humming as he sips his peach tea, trailing beside his brother on their walk home.

“‘Tsumu,” Osamu says, hands in his pockets, face turned up to the sky, “I’m not playing volleyball in college.”

It takes a while for the words to register; when they do, Atsumu stops short. He stares at his twin in stunned silence, straw falling out of his drink to clatter to the ground.

“You’re,” he falters. “Huh?”

Osamu shrugs, as if he’d simply said he was changing his mind about where to eat for dinner, and not making a life-changing decision for them both. “’M not playing volleyball anymore.”

Atsumu feels like he’s breathing in splinters.

“But,” he starts. Nothing follows.

( _But what about me,_ he wants to say, but it won’t come out.)

Osamu turns to him and smiles, the smile reserved for Atsumu and Atsumu alone. It’s not gentle, because there is nothing gentle about Osamu on or off a volleyball court, but it’s — it’s his. It belongs to Atsumu, just like the rest of the pieces of Osamu that the world doesn’t get to see.

Osamu smiles and says, “You’ve never needed it t’ be me on the volleyball court.”

It hits like the drop-off in noise when Atsumu holds up a hand before a serve. That’s how Atsumu feels — like abrupt silence. He looks at Osamu and thinks about how they were born for each other, and about Osamu at nine years old, agreeing to play volleyball with him.

He shoves the half-empty cup of peach tea into Osamu’s hands and then stalks off, leaving his twin behind.

(He forgives Osamu two weeks later, but not before he’s pieced his world back together, rearranged it around the empty spot on the volleyball court where he’d always thought his brother would be standing, ready for a run-up ahead of a spike.)

Osamu knows even at eighteen-turning-nineteen that Atsumu loves volleyball more than he does, so stepping down isn’t much of a sacrifice. Unlike Atsumu, there is more he wants to do in life than stand on a volleyball court. And this isn’t a criticism of his twin, no; it’s not something that makes either of them better or worse in the long run. It’s just one of their fundamental truths.

He also knows that Atsumu is the only person who won’t press him to change his mind because he’s _wasting his potential_ and _worked so hard for this long._ Atsumu won’t be disappointed for Osamu’s sake. His twin is selfish this way, in a way Osamu hates and adores.

Atsumu will be disappointed because his all-encompassing desire for volleyball includes Osamu, and now he has to adjust to a court without Osamu on it. But once his tantrum is over, Atsumu will understand.

“Gotten over yourself yet?” he asks one afternoon when Atsumu wanders into his room, half-dressed.

His answer is a Pokeball plushie sailing over to nail him squarely in the face, but Atsumu’s also carrying his grey bomber jacket with him as he leaves the room, so Osamu just laughs and goes back to his homework.

At nineteen, they’re living on their own in a tiny apartment in Sendai. Atsumu had taken the best offer to go pro with the MSBY Black Jackals, and reality has slapped him in the face with the fact that he’s not all that good as the world had made him out to be in high school. Osamu goes to Tohokudai, and now finds himself staggering under the weight of academics while trying to be an independent adult. He might have gone to other schools, other cities, but.

(There are still four fundamental truths between them, here, then, and always.)

“G’mornin’,” Atsumu mumbles as he stumbles into their tiny kitchen, and Osamu sips his coffee primly as he sticks out a foot to trip his half-asleep twin.

“Good morning,” he replies while Atsumu goes sprawling.

“Get fucked,” Atsumu grumbles into the floor.

They have an easy routine in the mornings. Osamu wakes up and makes coffee, sets out their breakfast things. If Atsumu isn’t up by the time the electric kettle has boiled, Osamu goes to kick him awake. He then proceeds to eat while his twin remembers how to function as a human being. Osamu gets first turn at the shower and usually doesn’t use up all their hot water. He leaves the house first, while Atsumu cleans up in the kitchen.

Sometimes Atsumu trails his fingers through Osamu’s hair as he squeezes his way over to the counter. Sometimes Osamu leaves out a peach or an apple for Atsumu to eat with his rice and fish. Sometimes they’re still in the same bed in the morning, and Osamu thinks about brushing a kiss over Atsumu’s temple before he gets up.

Intimacy is easy between them but gentle affection, less so. Still, Atsumu finds he likes the way they are now.

(When Osamu sneaks a hand under his shirt while he’s trying to cook dinner; when Osamu bites lightly at his shoulder after he’s cleaned up for bed; when Osamu lets him under the sheets and tucks his palm into the dip of Atsumu’s waist — yeah, Atsumu likes the way they are.)

The first time Atsumu sleeps with Osamu is entirely unplanned.

He comes home late on a Saturday afternoon, having spent the last three and a half days in a different city. They’d won the game by the skin of their teeth and Bokuto Koutaro’s powerful cross-court spike. Atsumu is still getting used to setting for Bokuto, who feels like an endless geyser of energy most days. He has no idea how Bokuto’s former setter had handled the boy’s demanding nature, but it exhilarates and exhausts him in equal measure.

Atsumu comes home and enters their living room to find Osamu sprawled out on the couch, shirtless and watching Code Geass on Netflix.

“’M home,” he says, and his twin reaches for another chip from the bag on his chest.

“Welcome back,” Osamu replies vaguely. Atsumu huffs and shakes his head.

His twin doesn’t move from his spot on the cushions while Atsumu drops his stuff off in his bedroom and goes to take a shower. He does roll his eyes when Atsumu comes into the living room while wearing Osamu’s ouroboros hoodie. Atsumu has to wrestle his twin into sitting up and making room for him on the couch, but soon they’re settled against each other, watching as Lelouch turns a ragtag group of would-be terrorists into a full-fledged resistance.

It’s halfway through the next episode when Osamu’s arm sneaks around Atsumu’s waist, fingers toying with the hem of the hoodie. The scattered brush of knuckles over skin starts winning more of Atsumu’s attention than the anime on screen, until a shiver starts creeping up Atsumu’s spine. He’s not sure what prompts him to move, in truth — the sensation of it; the building intimacy of the moment; the faint amusement tucked in the corners of Osamu’s eyes and mouth when Atsumu looks up to tell Osamu to _quit it._ But one moment Atsumu is exhaling shakily as Osamu’s pinky teases above the waistband of his boxers; the next he’s pushing up off the back of their couch and slinging his leg over his twin’s parted thighs.

“Can I help you?” Osamu asks casually, expression like butter wouldn’t melt.

“Get bent,” Atsumu snaps back, then kisses him.

It’s easy to get lost in this — the feel of Osamu’s mouth moving against his own, the subtle way Osamu falls back against the couch; the way his twin tips his head back just slightly, letting Atsumu assume control. Sometimes Atsumu idly thinks about all the lines they should never have crossed, but he also knows: _we were born for each other._ He’d followed Osamu into this world, not as two halves that form a complete whole, but as the only person against whom all of Osamu’s jagged edges would fit. There are no lines between them. There never have been.

So it’s easy, then, to shift and let Osamu settle wide palms on the jut of his hips. It’s easy to tilt his chin and let Osamu breathe open-mouthed over the thud of his pulse. It’s easy, it’s so—

“Oh _fuck_ you,” Atsumu gasps, half-laughing, as Osamu bites at his throat and squeezes his ass at the same time.

“Hmm.” Osamu hums mock-consideringly, nosing into the hollow of Atsumu’s shoulder. “Nah, no thanks.”

“ _Double_ fuck y—” Atsumu gets cut off with another kiss, clumsy in their snickering. He shoves at Osamu’s unmoving chest. “Could ya get a move on—”

He breaks into a moan as Osamu rolls his hips up and tightens his grip on Atsumu’s ass at the same time. They’ve never done things like this, honestly — hardly even come close a scant number of times — but it feels—

It feels like everything else about them. Simple, inevitable. 

Atsumu goes where Osamu leads, this time, rolling his hips to meet Osamu’s thrusts until they’re both panting and on edge. His twin’s hair is matted to his forehead; Atsumu reaches up and threads his fingers through the strands, using the leverage to push Osamu’s head back so they can kiss again. It’s too much and not enough; it’s too hot in their little apartment that has no airconditioning, but Atsumu wants closer and closer still. He wants to climb inside Osamu and settle into the spaces between his ribs and never come out.

From the way Osamu’s fingers dig into his ass, hard enough to leave marks, his twin no doubt feels the same.

“Fuck, ‘Tsumu,” Osamu groans, letting his head fall to the back of the couch. “Y’ feel so — _fuck._ ”

“Hah.” Atsumu tries to grin but it’s partially ruined by the strangled noise he makes when Osamu’s fingers drift lower, closer to the space between his legs. His twin’s cock is straining under his sweatpants, and Atsumu can feel it every time he grins his pelvis into Osamu’s. He reaches down, fumbling under the waistband and pleasantly surprised to find no underwear. He swallows the sound Osamu makes when Atsumu gets a hand around his cock.

They move like that — Atsumu fumbling his grip, Osamu using his fingers to play with Atsumu and get his twin off against his own thigh. They kiss clumsily, grazing teeth against whatever skin they can reach.

“Wanna—” Osamu grits his teeth as Atsumu twists his wrist and flicks his thumb over the head of Osamu’s cock. “Fuck, y’ gotta — lemme fuck you, so good, ‘Tsumu—”

“Yeah.” Atsumu doesn’t know what he’s agreeing to — that Osamu should fuck him, that it’ll feel good, that they need to do this again and will. “‘Samu, ‘m gonna—”

“Yeah—”

Atsumu comes first, shuddering and gasping and squeezing Osamu’s thigh between his own. A little later and he feels sticky cum all over his fingers as Osamu pitches forward and presses his teeth to Atsumu’s collarbone, groaning low in his throat.

For a few moments they simply sit there, Osamu collapsed back on the couch and Atsumu on top of him. Behind them, the anime runs on forgotten, sounds of explosions and fighting filtering in through the little bubble they’ve built up around each other. Eventually Osamu pries a hand from Atsumu’s skin and lifts it to his mouth, licking at his finger experimentally. The sight of his twin’s lips pursed around his slick pointer finger is one of the hottest things Atsumu’s ever seen, so he shoves Osamu’s hand away and kisses the taste of himself out of Osamu’s mouth.

“Like that, do ya,” Osamu teases when they part. Atsumu retaliates by biting his lip, then smearing cum onto his twin's chest. “Oi, fuck outta here—”

“I need to change,” Atsumu announces grumpily, trying to wriggle his way off Osamu’s laugh. His twin rolls his eyes and lets him up, but follows him into the bathroom that is too small to fit two still-growing boys. Osamu strips out of his pants while Atsumu undresses completely and shoves his hand under cold water. His twin leans over to bite Atsumu on the shoulder as he leaves the room, presumably to get new clothes.

“Asshole,” Atsumu grumbles, glowering at the mirror. 

Osamu just hums in response, but it feels like the words _yes, but I’m yours._

It is not an earth-shattering, pivotal moment. Atsumu just takes another shower while Osamu makes dinner, and comes out to find some clothes piled on top of the washing machine, waiting for him. He wanders into the kitchen and bothers Osamu while he cooks, sneaking bits and pieces every so often and getting whacked on the knuckles half the time. Osamu eats half his pickled radishes and Atsumu retaliates by taking the last pudding cup. A little later and Osamu throws an eraser at him for no reason.

They are nineteen, and sitting together in a cramped living room. Osamu spreads his schoolwork out on the low table and Atsumu lifts weights while continuing their Netflix marathon. He gets up for water and brings Osamu some barley tea. After a while, he ‘accidentally’ kicks Osamu in the hip as he makes his way to his room to sleep.

In the morning, they will wake up, and start the day all over again. 

That, too, is a fundamental truth between them.

**Author's Note:**

> thank you for reading!! we are all here to face god and walk back into hell together. feel free to drop by [@kinkykawa_](https://twitter.com/kinkykawa_) for more miyacest and other fun problematic things XD (finally edited that in forever after making that account lol)


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